1. Field of the Invention
The present invention relates, generally, to networks and methods for linearizing FM/CW radar systems and, more especially, to such networks and methods wherein the driving circuitry for the transmitter is controlled to provide a linear sweep of the interrogation signal. The present invention is adaptable for various uses, including applications in aircraft altimeters, autocollision avoidance radar systems and missile guidance systems.
2. Description of the Background Art
FM/CW radar systems are well known and enjoy many applications. FM/CW radar systems offer many advantages over pulse radar systems, which require interrogation signals having high peak power and a fairly precise time reference to obtain adequate target resolution; oftentimes also requiring elaborate precautions against errors which can arise due to variations in, e.g., pulse width of the interrogating signal in order to maintain acceptable target discrimination and/or resolution. On the contrary, FM/CW radar systems, with their frequency modulated continuous interrogation wave, make possible range resolution of a target simply as a function of frequency and further admit of the advantage of continuous measurement without time reference.
The foregoing theoretical advantages to the contrary notwithstanding, FM/CW radar systems are not without their indigenous problems. Crucial to the accurate resolution of a target or object field is the linearity of the sweep of the frequency of the transmitter signal. As these radar systems rely upon the mixing of the interrogation and reflection signals to achieve a beat or difference frequency indicative of range, any nonlinearity in the sweep of the transmitter frequency will manifest itself as a false or inaccurate indication. More specifically, where the sweep is linear and when the transmitter signal is then mixed with the receiver signal, the optimal result of a single value beat frequency representing range is achieved. However, where the transmitter signal is not swept linearly, the beat frequency achieved upon mixing will vary as a function of time proportional to the degree of nonlinearity. Hence, a point target will appear extended and the reliability of the system for target discrimination is reduced dramatically.
Prior approaches to linearizing these systems have focused on control of the transmitter sweep circuitry to provide the desired degree of resolution. Many have striven toward the control of the modulation of the continuous wave by the use of feedback loops associated with the transmitter circuitry. Sometimes the approaches are fairly simplistic, other times very complex.
Representative of certain prior art systems is that disclosed in U.S. Pat. No. 3,341,849. The patentees there are concerned about errors inhering in an FM/CW altimeter due to any inaccuracy in operation of the transmitter system. The principal approach to resolution of the problem is the continuous adjustment of the average frequency versus time relationship in the transmitter to insure accurate altitude indications during use and over the range of the instrument. This is achieved, in capsule sum, by monitoring the transmitter, applying a coupled signal as one input to a mixer and a delayed signal as another to generate an error signal proportional to any inaccuracy in the transmitter output; this error signal being utilized to control the transmitter modulation circuitry.
U.S. Pat. No. 4,008,475 discloses a stabilizing and calibrating circuit for FM/CW radar systems. The approach suggested there utilizes continuous feedback to the FM oscillator to account for, and eliminate, drift in the frequency excursions of the FM waveform controlling the transmitter. A control signal is derived by, inter alia, monitoring the transmitter signal and generating a delayed signal representative thereof. This delayed signal is mixed or beat against a signal from the VCO controlling the transmitter in order to obtain a difference signal used for calibration. The difference signal, having been suitably processed, is applied through a loop to control the oscillator.
U.S. Pat. No. 4,106,020 is also concerned with the problem of undesirable changes in FM modulation waveforms for an FM/CW radar. This approach differs conceptually from the foregoing, insofar as the results of variation are compensated rather than the cause of variation (i.e., nonlinearity) being controlled. The disclosed system utilizes a target-simulating delay line and a scaling network to compensate for undesirable deviations in the modulation waveform. In part, this is achieved by applying signals representative of a target to the counting terminal of a counter while a signal representative of a simulated target is coupled to the reset terminal of the counter through a divide-by-N circuit. If the modulation waveform changes, the counter is caused to reset sooner or later by a predetermined amount of time, thereby scaling the target data appropriately, based upon errors in the peak amplitude of the modulation waveform and/or its period. While this patented system discloses a type of correction, it is one simply predicated upon a scaling factor and does not address compensation for nonlinearity in transmitter sweep frequency.
U.S. Pat. No. 3,340,529 is interesting in its disclosure of an FM aircraft altimeter designed to reduce so-called "step errors", which arise where target range is in error by an integral number of cycles. Most remarkable about the approach suggested there is that it employs an intentional impression of a nonlinear phase change to overcome this "step error" problem. Thus, as opposed to addressing the problem of nonlinearity in the transmitter frequency sweep, such nonlinearity is intentionally created.
Of fairly broad or general applicability within the context of the present invention are U.S. Pat. No. 4,038,612 and No. 4,129,832; both of which relate to networks for generating linear frequency ramps used to control, e.g., a VCO. Each is further noteworthy for the inclusion of a memory circuit (e.g., a random access memory--"RAM") which operates upon the driving signal in a way to provide for linearity in the output. In the case of the '612 reference, the VCO responds to a controlled voltage ramp in which variations are detected by an error-sensing circuit at predetermined, accurately timed points. Discrete errors at plural points throughout the frequency ramp are computed and integrated over a plurality of sweep cycles and are thence employed as compensation for any nonlinearity. Zero crossings of a sampled signal characteristic of the swept oscillator output are determined and applied to a digital memory and integration circuit which stores samples over a number of cycles of the frequency ramp and integrates individual errors for each sample point over a plurality of sweeps. An error averaging process yields a plurality of correction signals which are fed to a digital-to-analog converter and, on further processing, make appropriate corrections to the ramp signal. As the patentees there briefly summarize, the approach may be thought of as a low-pass process wherein analog signal corrections modify the slope on an instantaneous basis at selected points within the ramp in order to effect linearization of the output. The '832 patent enjoys conceptual similarity, insofar as a programmable memory interfaces with a D to A converter in the control of a VCO with an eye toward linearization through periodic updating of the control circuitry based upon sampling of the output.
While the aforementioned patented systems broadly involve, and to varying degrees, FM/CW ranging systems (or like instruments) and problems associated with certain instabilities thereof (and principally nonlinearity), none discloses or suggests a network for accomplishing the objective of linearization in the face of varying sweep rates. That proves to be a substantial limitation on the adaptability of known systems, and particularly as respects airborne target seekers such as those associated with missile guidance. More specifically, FM/CW radar systems used in the guidance of a missile to a desired target employ variable sweep rates (respecting the frequency versus time relationship of the interrogation signal), which variation is a function of range from the missile to the desired target. Longer ranges imply lower slopes while the closer the approach the steeper the slope. A rationale behind the variation in sweep rate is the maintenance of a constant frequency intermediate frequency which is conveniently obtained by increasing the sweep rate the closer the missile approaches the desired target and the shorter the propagation time becomes. None of the systems discussed above adresses the linearization of the sweep of the transmitter signal of an FM/CW radar by segmented, sequential sampling thereof, and appropriate and parallel segmented, sequential correction in the face of varying (and perhaps widely so) sweep rates.
Accordingly, the need exists to provide an improved system for linearizing the sweep of the interrogation signal of an FM/CW radar; and particularly one where the sweep rate is a variable function over range. The need further exists to provide such a system which is of relatively low cost, compact and lightweight, and yet which is reliable in operation.